Can I replace the existing electric detectors with ones that detect carbon monoxide too?
Just to clarify, they all don't need to be one or the other, you can mix and match.
The advantage of a hardwired system is that any unit detecting any hazard that it is designed to detect alerts all of the units. So you can put (smoke +) CO detectors close to the potential sources of CO, where there will be the highest concentration. The interconnected units will already be providing a "sympathetic" alarm long before the CO concentration in more "remote" locations is sufficient to directly trigger a detector there.
So, for example, if the only potential source of CO in your house is a furnace and water heater in the basement, you can put a combination smoke+CO detector in the basement and smoke-only detectors in the other hardwired locations. If you're worried about the possibility of that single CO detector failing, you can always put a second combination unit as a "backup" in another strategic location, like the bedroom area.